Broadly speaking in a switch mode power supply a magnetic energy storage device such as a transformer or inductor is used to transfer power from an input side to an output side of the SMPS. A power switch switches power to the primary side of the energy storage device, during which period the current and magnetic field builds up linearly. When the switch is opened the magnetic field (and secondary side current) decreases substantially linearly as power is drawn by the load on the output side.
An SMPS may operate in either a discontinuous conduction mode (DCM) or in continuous conduction mode (CCM) or at the boundary of the two in a critical conduction mode. In this specification we are particularly concerned with DCM operating modes in which, when the switching device is turned off, the output voltage steadily, but gradually, declines until a point is reached on the knee of the output curve at which substantially zero output current flows an the inductor or transformer begins to ring, entering a so-called oscillatory phase. The period of the ringing is determined by the inductance and parasitic capacitance of the circuit. In this specification DCM includes so-called critical (discontinuous conduction) mode (CRM) operation in which the power switch is turned on again at the first trough of the oscillatory phase (sometimes referred to as the flyback oscillation). Operation in CRM can be particularly efficient by reducing losses associated with the power switch turn-off transition.
Often the output voltage of an SMPS is regulated by sensing circuitry on the output side, coupled back to the input side of the SMPS by means of an opto-isolator. However some improved techniques employ primary side sensing or, more generally, sensing employing an auxiliary winding on the magnetic energy storage device, or in some related circuits an auxiliary winding of an output filter inductor.
Some background prior art relating to primary side sensing can be found in U.S. Pat. No. 6,958,920; U.S. Pat. No. 6,721,192; US2002/015315; WO 2005/048442; WO 2004/051834; US2005/0024898; US2005/0169017; U.S. Pat. No. 6,956,750; U.S. Pat. No. 6,862,198; US 2006/0056204; U.S. Pat. No. 7,016,204; US 2006/0050539; US 2006/0055433; US 2006/0034102; U.S. Pat. No. 6,862,198; and U.S. Pat. No. 6,836,415. Still further background prior art can be found in U.S. Pat. No. 6,385,059, US20050276083, U.S. Pat. No. 6,977,824, U.S. Pat. No. 6,956,750, U.S. Pat. No. 6,900,995, WO2004082119, U.S. Pat. No. 6,972,969, WO03047079, U.S. Pat. No. 6,882,552, WO2004112227, US2005285587, WO2004112226, WO2005011095, U.S. Pat. No. 6,985,368, U.S. Pat. No. 7,027,312, U.S. Pat. No. 6,373,726, U.S. Pat. No. 4,672,516, U.S. Pat. No. 6,301,135, U.S. Pat. No. 6,707,283, and U.S. Pat. No. 6,333,624.
Referring now to FIG. 1, this shows an example of a switch mode power supply circuit with primary side sensing. The power supply comprises an AC mains input coupled to a bridge rectifier 14 to provide a DC supply to the input side of the power supply. This DC supply is switched across a primary winding 16 of a transformer 18 by means of a power switch 20, in this example an insulated gate bipolar transistor (IGBT). A secondary winding 22 of transformer 18 provides an AC output voltage which is rectified to provide a DC output 24, and an auxiliary winding 26 provides a feedback signal voltage proportionally to the voltage on secondary winding 22. This feedback signal provides an input to a control system 28, powered by the rectified mains. The control system provides a drive output 30 to the power switching device 20, modulating pulse width and/or pulse frequency to regulate the transfer of power through transformer 18, and hence the voltage of DC output 24. In embodiments the power switch 20 and controller 28 may be combined on a single power integrated circuit.
As can be seen, the primary side controlled SMPS of FIG. 1 derives feedback information from the primary side of the transformer, using an auxiliary winding to avoid high voltage signals, the voltage being stepped down by the turns ratio of the transformer. As the skilled person will appreciate, however, it is not necessary to employ a separate auxiliary winding although this may be convenient if such a winding is already contemplated to provide a low voltage supply to the controller. For example, a voltage of the primary winding may be sensed, preferably capacitor coupled so that it can be referenced to the ground of the controller, and stepped down using a potential divider. An example circuit for this is shown inset in FIG. 1, with a dashed connection to the primary winding 16. The skilled person will further appreciate that an auxiliary winding is not necessary to provide a dc supply for the controller as this may be derived from the high voltage dc supply on the primary side of the SMPS or in a number of other ways, for example using a capacitor charge pump driven via a diode from the switched voltage on the power switch. In some preferred implementations, therefore, the auxiliary winding is omitted.
We will describe techniques for using the auxiliary voltage waveform to generate feedback information for regulating an SMPS. In embodiments this facilitates operation across a wide range of input and output conditions. In U.S. Pat. No. 6,958,920 a relatively complex arrangement is employed to sample a feedback voltage from an auxilliary winding at a time Tfb, which is deliberately offset from a time (To) of zero magnetisation current. It is desirable to improve upon this arrangement.